Simply open up the .api file with any program that can handle the zip format and extract the install.rdf file. I find ark very easy to use myself, just open, drag, and drop. You don't even have to change the extension to .zip for it, as some other programs require.
Open install.rdf in a text editor and change the number in the <em:maxVersion>1.0</em:maxVersion> line. You can use the * wildcard if you want, say something like '1.*', so you don't have to redo it every time the minor number changes.
Finally return the edited install.rdf to the .api file and you're done. The extension should now install for you.
Whether it will still work or not is a different question, since new versions of the program might have changes that are incompatible with the old extensions. But at least you can try it out.
For future reference: